Filters, diplexers, multiplexers and other devices which incorporate an electromagnetic filter assembly disposed in an electromagnetic waveguide may be manufactured in the form of two blocks or plates with half of the waveguide duct machined in each plate and one or more a thin metal sheets or shims with appropriate cut-outs sandwiched between the plates. The cut-outs in the or each shim are produced in such a way that when the parts are assembled, the cut-outs form a series of thin metal bridges which traverse the waveguide duct. The axial thickness and spacing of the bridges, and the number of bridges, determine the frequency characteristics of the filter.
The current known art of tuning waveguides is to use screws which penetrate into the waveguide duct, positioned along the centre line of the waveguide. This achieves minimum insertion loss and maximum tuning effect. However, such an arrangement is not practical with metal shim filters since the tuning screws must be clear of the shim, and placing the screws clear of the shim increases the insertion loss of the filter. Furthermore, in devices which incorporate two or more filter assemblies use of tuning screws may require the tuning of each filter cavity and thus has the disadvantage of requiring skilled operators to carry out the task, which may be time consuming.
Another known way of tuning such devices is to form dents in the walls of the waveguide duct, but this has the disadvantage of being time consuming, difficult to achieve and irreversible.
The present invention seeks to provide a new and inventive form of tuning method for such devices, which does not involve any of the above methods and is inexpensive, fast, simple to achieve and manufacture, repeatable, low loss and reversible.